This invention relates to hydrodynamic-mechanical transmissions for motor vehicles wherein there is provided a torque converter which is selectively connected to a planetary gear arrangement with hydraulically actuable clutch members to provide the various transmission speeds.
In particular, the invention relates to such transmissions wherein the planetary gear arrangement includes a first sun gear which is connectable to the torque converter output turbine rotor by a first clutch and a second sun gear connectable to the turbine rotor by a second clutch. A planetary gear carrier is connectable by a third clutch to the input pump rotor of the torque converter. Mounted on the planetary gear carrier is a first planetary gear connected to the second sun gear and a ring gear. A second planetary gear is mounted on the same planetary carrier and connected between the first sun gear and the first planetary gear.
A transmission of this general type is described in published German Patent Application No. 1,625,124. In the transmission described in that reference, the first forward speed is obtained by actuating the first clutch to engage the first sun gear to the torque converter output turbine rotor and holding the planetary carrier using a one way clutch or brake. Reverse speed is similarly obtained by locking the planetary carrier in position with a brake and engaging the second clutch to connect the second sun gear to the output rotor of the torque converter. Second gear is obtained by actuating the first clutch and locking a second brake which fixes the second sun gear in position. In order to obtain a direct drive for third gear, the first and third clutches are engaged which locks the planetary gear system into a single rotating set of gears. A fourth gear may be obtained by engaging the third clutch and the second brake, so that power is provided directly from the torque converter input to the planetary carrier which rotates around the locked second sun gear.
When the third clutch is engaged, there is provided a direct mechanical connection between the engine and the planetary gear system bypassing the torque converter, which usually operates with some slippage and consequent inefficiency. The planetary gear carrier is directly connected to the input pump rotor which is mounted on the engine drive shaft. Thus, in the fourth speed, or overdrive, the torque converter is completely bypassed. Against that, in third speed, there takes place a power branching because part of the power is transmitted to the planetary gear through the torque converter over the first clutch and the remaining power over the third clutch bypassing the torque converter.
While elimination of the torque converter from the transmission drive train eliminates losses on account of slippage and increases overall efficiency, the direct mechanical connection of the drive shaft to the engine causes a direct transmission of engine torque irregularities to the drive wheels of the vehicle. Especially at low engine and vehicle speeds, the engine torque irregularities can cause a perceptible impairment of riding comfort.
It is an object of the present invention to provide a hydrodynamic mechanical transmission of the known type, but having improved riding comfort, particularly at the lower speed range of third gear.